Computing technology continues to become less expensive while providing more capability. Physical dimensions have shrunk to meet portability goals. Power consumption should be minimized, especially in portable devices that require portable power supplies, e.g., batteries.
High power requirements of conventional memory devices, e.g., hard disk drives, limit long-term battery operation. Microprocessors consume small amounts of power compared to such ancillary devices. Higher speed devices are also desirable. Criticism has been voiced in the trade press about the inability of mass storage devices such as disk drives, CD-ROMs, and DVD drives, to name a few, to keep up with the advancing speed of the microprocessors.
Electron emitters that create electron beams present the basis for alternative possible solutions for memories, electronic chip fabrication equipment, and other devices. Electron beam technology has been present for many years in consumer products such as television (TV) tubes and computer monitors. These devices use what is known as hot cathode electrodes to create a source of electrons that are directed to and focused on the viewing screen. These hot cathode emitters are ill-suited for computer scale devices due their large size, high temperature and high power consumption. The trend, even in television, is to move toward a more compact solution. Plasma and LCD televisions are examples of the trend away from the bulky, hot cathode technology.
While research has taken place in a number of new technological fields, the field of cold cathode electron emitters such as tip and flat emitters has attracted the attention of many manufacturers. Several problems exist in converting this cold cathode technology to products. One such problem is the creation of an electron focusing structure that can be used in multiple applications that require a high density of emitting devices such as with mass storage devices.
A typical tip or flat emitter driven memory device is based upon the controlled use of electron emissions from an emitter. An emitter emits electrons in response to an electrical signal. Controlled emissions form a basis to create useful electrical and optical effects. Focused emissions can affect various media to produce, for example, memory and lithography effects. These and other applications require the use of controlled and focused electron beams. Production of such beams involves the fabrication of an emitter and focusing structure, typically an electrostatic lens.
Various emitter driven devices make use of a target anode medium. The target anode medium is the focus point for the controlled emissions. A target anode medium is held at hundreds of volts differential from the emitter/cathode structure. Alignment and focusing length are important issues in emitter driven devices. Fabrication of lenses on emitter chips requires the precise alignment of the emitters and focusing elements. To achieve alignment, standard practice for micro-fabricated emitters is to form the entire lens and emitter structure in a single self-aligned photostep. This achieves good lens/emitter alignment, but fixes the distance of the lens from the emitter and also limits the thickness of the lens. Generally, the lens is the same distance from an extractor as the extractor is from a tip emitter formed in a well. The focusing length is accordingly limited to the short distance afforded by the separation of various metal layers in an emitter/focusing lens chip.
The single self-aligned photostep process further sets the diameter of the lens to that of the well; since both are formed from the same etch. Due to the common size of the extractor and lens and their relative positions, the divergence angle of the emission beam from the emitter is wider than the lens. This adversely affects the ability to produce tightly focused spots from the emissions. A tightly focused spot size, e.g., less than 35 nm, is desirable to increase density of a memory and a narrowly diverging beam is desirable for a scientific instrument or a lithography tool. Some conventional devices have achieved spot sizes of about 40 nm by using apertures to block a substantial amount of stray emissions. Efficiency, as measured by the percentage of electron emissions that are used to produce the focused spot, is accordingly low. Significant aperturing to reduce spot size can reduce emission efficiency 100× to 10,000×. Other approaches to reduce spot size include the use of multiple lenses and high acceleration voltages. High voltages conflict with power consumption and may not be available in certain portable devices. Complex lensing can raise manufacturing costs, and may be difficult to implement in a high volume manufacturing process.
The conventional well housing a tip emitter also is deep. The standard small lens size requires an extremely precise alignment, ˜0.04 μm, between the tip/extractor and lens. As mentioned, a single etch produces the well and lens to achieve the alignment, and aspect ratios of the well are generally high, about 2 to 1 (depth to diameter). This creates processing complexity because it is harder to deposit a tip emitter in a deep well as the coating of the inside of the well is to be avoided while forming the emitter tip. This may require more sophisticated tooling, e.g., larger evaporators. Deep wells can also produce poor emitter tip quality and low yields.